Absolutely the GPU temperature can influence your CPU temperature, especially when running GPU stress tests. From the physics standpoint............ heat rises, so when you're busting your GPU with a stress test the heat from the GPU rises (assuming that your case is in a standard configuration). That heat rises right into the CPU cooler (assuming that you're running an air cooling solution on the CPU). Then at some point the entire interior of your case is heat soaked from the GPU stress test and overcomes the ability of the case to exhaust that hot air.
Think of it like this......... if you take a frozen bottle of water and place it into a 180 degree box, the interior of the box may get a little cooler, but your ice water is going to melt regardless.
Most GPU card cooling solutions cannot exhaust all of the heat generated out the back of the case, so it has to go somewhere. And even when you're running a GPU stress test your CPU isn't sitting there totally idle so it's generating heat of it's own as well.